# Tags historical scan class

**URL:** <https://forum.inductiveautomation.com/t/tags-historical-scan-class/14200>\
**Category:** Ignition\
**Tags:** ignition79\
**Created:** [May 3, 2017, 2:22pm UTC](https://forum.inductiveautomation.com/t/tags-historical-scan-class/14200 "2017-05-03T14:22:25Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![pgmo](https://avatars.discourse-cdn.com/v4/letter/p/d07c76/32.png) [@pgmo](https://forum.inductiveautomation.com/u/pgmo)\
**Post date:** [May 3, 2017, 2:22pm UTC](https://forum.inductiveautomation.com/t/tags-historical-scan-class/14200/1 "2017-05-03T14:22:25Z")

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I’m trying to figure out what the difference is between selecting the Historical Scanclass **Evaluate On Change** option, rather than selecting a **specific scan class** : if I use the _Evaluate On Change_ option, am I going to poll the tag value with the tag non-historical scan class?

Thanks in advance for any suggestion

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**Author:** ![matt.eaton](https://sea1.discourse-cdn.com/inductiveautomation/user_avatar/forum.inductiveautomation.com/matt.eaton/32/72046_2.png) [@matt.eaton](https://forum.inductiveautomation.com/u/matt.eaton)\
**Post date:** [May 3, 2017, 5:38pm UTC](https://forum.inductiveautomation.com/t/tags-historical-scan-class/14200/2 "2017-05-03T17:38:34Z")

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Yes, with the added caveat that it will not run if the tag does not change.

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**Author:** ![pgmo](https://avatars.discourse-cdn.com/v4/letter/p/d07c76/32.png) [@pgmo](https://forum.inductiveautomation.com/u/pgmo)\
**Post date:** [May 3, 2017, 8:02pm UTC](https://forum.inductiveautomation.com/t/tags-historical-scan-class/14200/3 "2017-05-03T20:02:15Z")

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Ok, thanks matt.eaton.

Regards